So, I've seen this a few places now.. when a floor collapses in a dungeon, or you take a portal up a level where you can still see and access the level below (such as the final fight in Markarth DLC)...
But companions will not follow... and inevitably will just be killed by environment, or adds that they don't even fight back because aren't on the same level...
Here's an example in Selene's Web when you fall through the web on final boss... Mirri became floor dps for the remainder of the fight...
https://www.youtube.com/watch?v=KEhzvsAkpAU